zeroing out part of an SDcard

[Robert Moskowitz]

On 08/21/2014 09:07 AM, Robert P. J. Day wrote:
> On Thu, 21 Aug 2014, Robert Moskowitz wrote:
>
>> More on my compressing images from SDcards.
>>
>> I only need the boot info and 1st partition, so I remove the other partitions,
>> but of course there is still 1s out there so it will not compress efficiently.
>> I want to zero out the end of the card so first I use fdisk:
>>
>> # fdisk -l /dev/sdb
>>
>> Disk /dev/sdb: 7.4 GiB, 7969177600 bytes, 15564800 sectors
>> Units: sectors of 1 * 512 = 512 bytes
>> Sector size (logical/physical): 512 bytes / 512 bytes
>> I/O size (minimum/optimal): 512 bytes / 512 bytes
>> Disklabel type: dos
>> Disk identifier: 0x0009e2ad
>>
>> Device Boot Start End Blocks Id System
>> /dev/sdb1 8192 1007615 499712 83 Linux
>>
>> Then dd:
>>
>> # dd if=/dev/zero of=/dev/sdb bs=1M seek=1007620
>> dd: ‘/dev/sdb’: cannot seek: Invalid argument
>> 0+0 records in
>> 0+0 records out
>> 0 bytes (0 B) copied, 0.000652852 s, 0.0 kB/s
>>
>> ======================
>>
>> seems you cannot seek on an SDcard. How do I do this? Since the change in
>> partition 1 is trivial, if I have to start again from an SDcard with all 3
>> paritions, I can do that...
>    it seems like you're trying to seek more than a terabyte into that
> card.

Well, man says about seek:

        seek=N skip N obs-sized blocks at start of output

and obs defaults to 512 bytes, and I *THOUGHT* fdisk was reporting # 512 
blocks, not bytes.